Micromechanical systems, which are frequently also known as microsystems or MEMS (microelectromechanical systems), are being increasingly used, in particular due to their small size, their relatively low price and their high reliability. This relates to, for example, the use of micromechanical systems as actuators or sensors, for example, in the form of sensors for detecting acoustic emissions, structure-borne noise sensors, acceleration and inclination sensors, rotational frequency sensors or pressure sensors.
“Acoustic emissions”, which are also known by the abbreviation “AE”, are usually understood to mean a phenomenon with which elastic waves are generated by impulsive excitation due to a sudden release of energy within a solid body. Corresponding acoustic emission signals, which are propagated in the form of structure-borne noise in the solid body, usually occur in a frequency range from about 20 kHz to about 1 MHz. Hereby, acoustic emission signals are very sensitive with respect to mechanical damage to a solid body or an object. For this reason, micromechanical sensors for detecting acoustic emissions—acoustic emission sensors—are used in particular for monitoring wear on mechanical parts such as, for example, anti-friction bearings. Sensors of this type usually comprise a system that can vibrate with a seismic mass suspended or fastened on spring elements. External forces or accelerations cause a deflection of the seismic mass with respect to fixed anchor points, in the form of, for example, fixed suspensions. This relative movement is evaluated, wherein frequently a capacitive principle is used to obtain the signals. Hereby, the seismic mass comprises electrode arrangements, which can, for example, have a comb-like design and, together with a fixed counter-electrode, form a variable capacity. Hereby, the determination of the value of the capacity or a change thereto enables the detection of acoustic emissions.
A micromechanical system with a system that can vibrate, comprises a seismic mass and at least two spring elements, wherein the spring elements are respectively fastened on one side externally to the seismic mass and on the other side to fixed anchor points of the micromechanical system such that the seismic mass can vibrate in a movement direction. Hereby, “in a movement direction” means that the system that can vibrate has precisely one linear degree of freedom, i.e. that a movement of the system that can vibrate during the operation of the micromechanical system is provided in precisely one direction only, the movement direction.
A micromechanical system of this kind is known from the chapter “System-Level Synthesis” in “Optimal Synthesis Method for MEMS”, Ananthasuresh, G. K. (Ed.), Springer, 2003, page 297-316.
There is currently a trend toward ever higher natural frequencies of micromechanical systems. This applies to both broadband applications and resonant, i.e. resonantly driven, systems. In addition to the aforementioned sensors for detecting acoustic emissions, further examples mentioned here are micromechanical filters and mixers for the high-frequency range.
Micromechanical systems with systems that can vibrate are frequently operated in the range of a resonant frequency, i.e. in the range of a normal mode determined by a corresponding natural frequency. Hereby, the frequency or the vibration shape associated with this frequency, for which the system that can vibrate is embodied or provided with respect to its operation, is also known as the useful mode. Hence, with a resonant operation, the useful mode is a normal mode, as a rule the first normal mode, of the system that can vibrate. Hereby, the vibration modes of higher natural frequencies can result in unwanted effects, such as, for example, superimposed interference signals or reduced sensitivity. For this reason, generally a larger frequency spacing, i.e. a mode separation, between the useful mode and the further vibration modes, in particular the other normal modes, of the system that can vibrate is desirable.
With resonant micromechanical systems, such as those used, for example, for sensors for detecting acoustic emission signals, there is usually a requirement for high stiffness of the system that can vibrate in the movement direction or useful direction. In addition, in order to avoid movements directed out of the envisaged movement direction, as a rule, again a much higher stiffness transverse to the movement direction is desirable. On the other hand, large-surface electrodes or electrode systems are required to achieve sufficiently high sensitivity of the micromechanical system. However, electrodes of this kind result in an influence of the dynamic response of the micromechanical system and there is a risk that the other normal modes of the system that can vibrate may occur in a relatively small frequency spacing relative to the useful mode (which, as already mentioned above, as a rule, corresponds to the main mode, i.e. the first normal mode). Hereby, the preferred direction of corresponding higher vibration modes can both lie within the plane clamped by the micromechanical system and also extend outside this plane. Vibration modes of the last-named type occur in particular with two-dimensional structures and are known as “membrane modes” or “out-of-plane modes”.
In principle, one measure to enlarge the frequency spacing between the vibration modes of the system that can vibrate of the micromechanical system can include enlarging the structural thickness of the micromechanical system. However, this is not simple to achieve with the known dry etching processes. In addition, this would only enable an improvement of the separation of the “out-of-plane modes” from the “in-plane modes” of the system that can vibrate.
It is also known from the aforementioned publication “System-Level Synthesis” that an improvement of the spacing between normal modes of a system that can vibrate can be achieved by reducing the number of electrodes. However, this has the drawback that, as a rule, this simultaneously significantly reduces the sensitivity of the micromechanical system.